Deformation Of Nematic Liquid Crystal Research Articles

Defect structures in nematic liquid crystals around charged particles

We numerically study the orientation deformations in nematic liquid crystals around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous electric field. If the dielectric anisotropy epsilon 1 is positive, Saturn-ring defects are formed around the particles. For epsilon 1< 0 , novel "ansa" defects appear, which are disclination lines with their ends on the particle surface. We find unique defect structures around two charged particles. To lower the free energy, oppositely charged particle pairs tend to be aligned in the parallel direction for epsilon 1> 0 and in the perpendicular plane for epsilon 1< 0 with respect to the background director. For identically charged pairs the preferred directions for epsilon 1> 0 and epsilon 1< 0 are exchanged. We also examine competition between the charge-induced anchoring and the short-range anchoring. If the short-range anchoring is sufficiently strong, it can be effective in the vicinity of the surface, while the director orientation is governed by the long-range electrostatic interaction far from the surface.

Exact tilt angle profiles for splay–bend deformations in nematic liquid crystals

The exact tilt angle profiles for splay–bend deformations, in nematic liquid crystal samples limited by inhomogeneous surfaces, are determined in the one‐constant approximation. The boundary value problem concerning the situation of strong anchoring at the surfaces of a sample of slab shape of thickness d (Dirichlet's problem) is analytically solved in the presence of an external uniform field. The boundary value problem concerning the weak anchoring situation (mixed problem) is also exactly solved in the absence of an external field. The results are used to obtain the thickness dependence of the optical path difference between the ordinary and extraordinary rays, from which the physical properties of the sample can be deduced.

Dynamical behavior of the director field for splay-bend deformations in nematic liquid crystals

The exact dynamical evolution of the director field for splay-bend deformations, in nematic liquid crystal samples limited by inhomogeneous surfaces, is determined in the one-constant approximation. The initial conditions and boundary-value problem concerning the situation of strong anchoring at the surfaces of a sample of slab shape of thickness d is analytically solved in the presence of a time dependent external electric field, and taking into account the viscous torque. The results are used to analytically obtain the time dependence of the phase shift between the two components of a linearly polarized beam impinging perpendicularly on the sample. The analysis can be relevant to investigate the phase retardation of a nematic cell submitted to an external voltage which is lower than or in the order of the Féedericksz threshold to induce deformations in the sample.

Deformation of nematic liquid crystals in an electric field

The behaviour of liquid crystal materials used in display devices is discussed. The underlying continuum theory developed by Frank, Ericksen and Leslie for describing this behaviour is reviewed. Particular attention is paid to the approximations and extensions relevant to existing device technology areas where mathematical analysis would aid device development. To illustrate some of the special behaviour of liquid crystals and in order to demonstrate the techniques employed, the specific case of a nematic liquid crystal held between two parallel electrical conductors is considered. It has long been known that there is a critical voltage below which the internal elastic strength of the liquid crystal exceeds the electric forces and hence the system remains undeformed from its base state. This bifurcation behaviour is called the Freedericksz transition. Conventional analytic analysis of this problem normally considers a magnetic, rather than electric, field or a near-transition voltage since in these cases the electromagnetic field structure decouples from the rest of the problem. Here we consider more practical situations where the electromagnetic field interacts with the liquid crystal deformation. Assuming strong anchoring at surfaces and a one dimensional deformation, three nondimensional parameters are identified. These relate to the applied voltage, the anisotropy of the electrical permittivity of the liquid crystal, and to the anisotropy of the elastic stiffness of the liquid crystal. The analysis uses asymptotic methods to determine the solution in a numerous of different regimes defined by physically relevant limiting cases of the parameters. In particular, results are presented showing the delicate balance between an anisotropic material trying to push the electric field away from regions of large deformation and the deformation trying to be maximum in regions of high electric field.

On the analogy between the field-induced and flow-induced deformations in nematic liquid crystals

The analogy between the field-induced and flow-induced deformations of nematic liquid crystals is summarized. The threshold onset of the deformation occurring under rigid boundary conditions, the unwinding of the helical axis in a chiral nematic material, and the saturation threshold appearing for weak anchoring are described as examples of analogous phenomena which take place both in external fields and during shear flow. The analogy has allowed prediction of a new threshold effect possible in a sheared, weakly anchored nematic. The description of this effect is supported by numerical calculations. Stability analysis was also performed.

Subsurface deformations in nematic liquid crystals: The hexagonal lattice approach

In this paper the existence of subsurface deformations in the orientational ordering of a nematic liquid crystal confined to a space between two planar substrates is studied by means of a lattice model. A superposition of the anisotropic induced-dipole--induced-dipole and isotropic Maier-Saupe interaction laws is used to describe intermolecular interactions in the nematic phase. To model the nematic phase we use a simple-hexagonal lattice, which does not introduce any bulk easy axes. The interaction nematic-confining surface (external anchoring) is described with the Rapini-Papoular form. We show that there is always a subsurface deformation in a layer of a few molecules when the intermolecular interaction law contains a nonzero fraction of the anisotropic interaction. The deformation is ascribed to the competition of external and effective intrinsic anchoring arising from the incomplete intermolecular interaction close to the surface. We also estimate the extrapolation length ${l}_{i}$ as a measure of the intrinsic anchoring. This length turns out to be of molecular dimensions for the pronounced induced-dipole--induced-dipole character of the interaction. However, the corresponding subsurface deformation strength, given by the normal derivative of the director component, is considerably smaller than ${1/l}_{i},$ as the simplest estimate suggests. A realistic ${l}_{i}$ can be achieved in our model only when the interaction anisotropy is considerably decreased.

The kinetics of non-linear orientational deformations in nematic liquid crystals in a uniform magnetic field

The non-linear kinetic equation describing the evolution of the orientational structural of nematic liquid crystals is investigated. The equilibrium structures are described and their stability and the behaviour of the solutions for long times are investigated. The super-slow evolution (for example, in a very intense magnetic field) of the irregular structure to a regular stable structure is described.

Magnetic Field Induced Deformations in Nematic Liquid Crystals

Abstract Unlike most organic materials, liquid crystals respond readily to magnetic fields. This response originates in the anisotropic magnetic susceptibility of nematics, and the relative ease with which they undergo orientational deformations. Because of the large optical birefringence of liquid crystals, these orientational deformations are easy to observe. An interesting demonstration is to place a small rare earth magnet on a nematic cell between crossed polarizers which is illuminated from below. As the magnetic field reorients the liquid crystal, beautiful interference colours appear, indicating director deformations.

Subsurface deformations in nematic liquid crystals.

The existence of subsurface deformations in a nematic liquid crystal sample of finite thickness is considered from a molecular point of view. A lattice approximation is used to take into account the intermolecular interactions responsible for the nematic phase. The analysis shows that for the Maier-Saupe interaction law the director profile is a smooth function in the whole sample. In contrast, in the framework of our simple model, for the induced-dipole--induced-dipole interaction law the director profile presents a large subsurface deformation. The results of our calculation are compared with the elastic theories for nematic liquid crystals recently proposed. In particular it is shown that the intermolecular interactions responsible for the effective splay-bend elastic constant are the origin for the subsurface discontinuity in pretilted nematic liquid crystal samples. Our predictions are obviously limited by the applicability of the na\"{\i}ve model considered in the present analysis. \textcopyright{} 1996 The American Physical Society.

New results of field-induced deformation of nematic liquid crystals for testing of digital integrated circuits

This paper deals with a homeotropic oriented nematic liquid crystal between the line structures of an integrated circuit and a backplate electrode. It is demonstrated, that the deformation and the spatial resolution essentially depend on the voltage of the backplate electrode and the distance between the lines. Experimental results will be presented and discussed by a simple model. For testing an integrated circuit a new simple method of preparation with a specially developed manipulator is presented. Based on a controlled backplate electrode, a novel analysis technique allowing the determination of logical states and digital operation is presented.

One- and two-dimensional deformations in nematic liquid crystals: Influence of the flexoelectric polarization

The elastic deformations in nematic liquid crystals are analysed in one and two dimensions. The influence of the flexoelectric polarization on the elastic properties of the medium is studied. It is shown that for one-dimensional deformations, the presence of the flexoelectric polarization introduces a renormalization of the elastic constants. On the contrary in the bidimensional case, the effect of the flexoelectricity is not a simple renormalization of the elastic constants. The profiles of the electric potential and of the tilt angle are evaluated for the cases in which the substrate is an insulating or conducting material. The validity of the calculations is discussed by considering the value of Debye's screening length of the nematic liquid crystal.

Out of shear plane deformations in nematic liquid crystals

Out of shear plane deformations in nematic liquid crystals

Deformation of Nematic Liquid Crystals in Electric and Magnetic Fields as seen by Proton Nuclear Magnetic Resonance

Abstract We have calculated the deformation of the director field within a nematic liquid-crystal cell when both extenal electric and magnetic fields are present. It can be proved experimentally that the line shape of proton nuclear magnetic resonance is strongly affected by the orientational distribution of the director. The analysis of the NMR-line shape of LC-sandwich cells in this manner can yield various parameters relating to the liquid crystalline material.

Controlled decay of electrically induced deformations in nematic liquid crystals

The transient behavior of electrically induced deformations of parallel oriented nematic liquid crystal layers was investigated. The nematic material used was a mixture of disubstituted benzoyloxybenzoic acid esters the dielectric anisotropy of which is positive at dc and low frequencies, and changes sign in the kHz range. The decay time of a deformation induced by a low frequency field can be considerably reduced by switching the frequency to the range where the dielectric anisotropy is negative. This effect becomes even more pronounced for higher viscosities.

Deformation of Nematic Liquid Crystals in an Electric Field

Abstract The problem of a nematic liquid crystal in an electric field is solved. Effects due to conductivity of the liquid have been neglected. Numerical solutions are given for various values of parameters. The solution is compared with experimental data taken from the literature. A computer program has been developed to do a non-linear least-squares fit of experimental data with the solution given in the present paper.

Electric-Field-Induced Orientational Deformation of Nematic Liquid Crystals: Tunable Birefringence

Spatially uniform birefringences tunable over the range 0. 0–0. 15 for applied voltages less than 20 V rms and with a sharp threshold below 4 V rms are obtained for thin layers of nematic liquid crystals with negative dielectric anisotropy. The achievement of spatially uniform tunable birefringence with negative-dielectric-anisotropy materials requires a source of in-plane anisotropy which is provided by a unique previously unreported molecular configuration at zero field. At V=0, the molecules align with their long axes at a small angle to the sample normal. The experimental results are analyzed in terms of a continuum model and experimental values obtained for the relevant elastic constants.

Deformation of Nematic Liquid Crystals with Vertical Orientation in Electrical Fields

A new electro-optical effect in nematic liquid crystals with negative dielectric anisotropy is described, using sandwich cells with molecular orientation perpendicular to the electrode surface (homeotropic alignment). In an arrangement between crossed polarizers, high-contrast ratios have been achieved with low voltages. This effect shows a high degree of non-linearity and thus permits the development of matrix displays without using additional non-linear elements.